Feature dimensions in semiconductor menufacturing are continually becoming smaller. Present manufacturing processes are at roughly 0.8 .mu.m. The next generation plans are to 0.5 .mu.m followed by 0.35 .mu.m within the next five years or so and are expected to reach about 0.15 .mu.m critical dimensions (CD) thereafter. While substantial effort is being expended on lithographic techniques for producing these very small feature sizes, considerably less effort has been devoted to the equally important issues of alignment and overlay between mask levels. Generally, registration of two structures to within 1/3 to 1/5 of CD is required for successful manufacturing.
In connection with the present invention, the term "alignment" is used to refer to the process of assuring reticle-to-wafer registration when the wafer is in the exposure tool. The term "overlay" refers to after-exposure measure of how accurately the process was carried out. More generally, "overlay" may also refer to the overall success of registering full patterns of the two mask levels and not merely or only the alignment marks.
An important distinction between "alignnment" and "overlay" is one of time: "alignment" occurs in the stepper before the upper level exposure is carried out; "overlay" is a measure of success in the alignment process after the upper level has been exposed and developed.
Alignment and overlay measurement techniques have been identified in the industry as substantial problems which pose very substantial limits on the progress in developing future generations of integrated circuits.
Most alignment techniques are proprietary. An exception is the alignment system used by Phillips/ASM [Performance of a wafer stepper with automatic intra-die registration correction, M. A. van den Brink, S.Wittekoek, H. F. D. Linders, F. J. van Hout, and R. A. George, SPIE 772, Optical Microlithography VI (1987)]. In this technique, which forms the basis of the alignment technique implemented in Phillips/ASM steppers, an incident laser beam (HeNe laser at 633 nm) is diffracted from a phase grating on the wafer (16-.mu.m period 400.times.80-.mu.m.sup.2 overall size) and imaged onto a second grating on the reticle. The intensity of the odd diffraction orders transmitted through the reticle, isolated by a spatial filter, is detected as the alignment signal.
Many other steppers rely on a small number of alignment marks, often in a nested L pattern that are imaged from the reticle to the wafer. It is important to emphasize that alignment will likely remain an electromagnetic process using optical or uv photons--and hence limited by diffraction effects.
Overlay measurement is often accomplished using a box-within-a-box technique. In this technique, a rectangular box on the reticle is exposed on a similar, but larger (or smaller) box on the wafer. Overlay is then measured by comparing the dimensions between the boxes on opposite sides, i.e., by measuring how well centerec the smaller box is inside the larger one. Provided both structures are uniform, this eliminates questions of edge definition as long as a consistent threshold is maintained. This technique depends on optical microscopy and is thus ultimately limited by the inevitable diffraction effects that become more severe as dimensions much less than a wavelength become important. See, for example, W. S. Ruska, Microelectronic Processing, McGraw-Hill, NY, 1987.